This invention relates generally to compositions of matter suitable for use in aggressive, high-temperature gas turbine environments, and articles made therefrom.
Nickel-based superalloys are used extensively throughout the turbomachines in turbine blade, nozzle, and shroud applications. Turbomachine designs for improved engine performance demand alloys with increasingly higher temperature capability, primarily in the form of improved creep strength (creep resistance). Alloys with increased amounts of solid solution strengthening elements such as Ta, W, Re, and Mo, which also provide improved creep resistance, generally exhibit decreased phase stability, increased density, and lower environmental resistance. Recently, thermal-mechanical fatigue (TMF) resistance has been a limiting design criterion for turbine components. Temperature gradients create cyclic thermally induced strains that promote damage by a complex combination of creep, fatigue, and oxidation. Directionally solidified superalloys have not historically been developed for cyclic damage resistance. However, increased cyclic damage resistance is desired for improved engine efficiency.
Superalloys may be classified into four generations based on similarities in alloy compositions and high temperature mechanical properties. So-called first generation superalloys contain no rhenium. Second generation superalloys typically contain about three weight percent rhenium. Third generation superalloys are designed to increase the temperature capability and creep resistance by raising the refractory metal content and lowering the chromium level. Exemplary alloys have rhenium levels of about 5.5 weight percent and chromium levels in the 2-4 weight percent range. Fourth and fifth generation alloys include increased levels of rhenium and other refractory metals, such as ruthenium.
Second generation alloys are not exceptionally strong, although they have relatively stable microstructures. Third and fourth generation alloys have improved strength due to the addition of high levels of refractory metals. For example, these alloys include high levels of tungsten, rhenium, and ruthenium. These refractory metals have densities that are much higher than that of the nickel base, so their addition increases the overall alloy density. For example, fourth generation alloys may be about 6% heavier than second generation alloys. The increased weight and cost of these alloys limit their use to only specialized applications. Third and fourth generation alloys are also limited by microstructural instabilities, which can impact long-term mechanical properties.
Each subsequent generation of alloys was developed in an effort to improve the creep strength and temperature capability of the prior generation. For example, third generation superalloys provide a 50° F. (about 28° C.) improvement in creep capability relative to second generation superalloys. Fourth and fifth generation superalloys offer a further improvement in creep strength achieved by high levels of solid solution strengthening elements such as rhenium, tungsten, tantalum, molybdenum and the addition of ruthenium.
As the creep capability of directionally solidified superalloys has improved over the generations, the continuous-cycle fatigue resistance and the hold-time cyclic damage resistance have also improved. These improvements in rupture and fatigue strength have been accompanied by an increase in alloy density and cost, as noted above. In addition, there is a microstructural and environmental penalty for continuing to increase the amount of refractory elements in directionally solidified superalloys. For example, third generation superalloys are less stable with respect to topological close-packed phases (TCP) and tend to form a secondary reaction zone (SRZ). The lower levels of chromium, necessary to maintain sufficient microstructural stability, results in decreased environmental resistance in the subsequent generations of superalloys.
Cyclic damage resistance is quantified by hold time or sustained-peak low cycle fatigue (SPLCF) testing, which is an important property requirement for single crystal turbine blade alloys. The third and fourth generation superalloys have the disadvantages of high density, high cost due to the presence of rhenium and ruthenium, microstructural instability under coated condition (SRZ formation), and inadequate SPLCF lives.
Accordingly, it is desirable to provide superalloy compositions that contain less rhenium and ruthenium, have longer SPLCF lives, and have improved microstructural stability through less SRZ formation, while maintaining adequate creep and oxidation resistance.